1. Field of the Invention
This invention relates to an improvement on a method for producing a silicon nitride dominating ceramics by using polysilazane as a precursor.
2. Description of the Prior Art
A so-called precursor method in which heat-resistant ceramics are produced by pyrolysis of organometallic polymers has advantages such as (1) the capability of forming products of special shape which can not obtained through conventional processings from ceramics powder (2) the ready attainment of products having A purity higher than those obtained through conventional processes, and (3) the capability of being processed at a lower temperature.
For example, a method is known in which the green fibers obtained by the melt-spinning of polycarbosilane is subjected to infusibilizing in air at about 200.degree. C. and thereafter heating at 1300.degree. C. in the atmosphere of an inert gas in order to obtain ceramic fibers containing, as a principal material, silicon carbide (Yajima; Chemistry Letters, p.551-554, 1976). Further, a method is known for producing ceramic fibers consisting of a mixture of silicon carbide and silicon nitride, which method involves the heating the green fibers obtained by the melt-spinning of polysilazane, in wet air at a temperature of 110.degree.-170.degree. C. to effect infusibilization and thereafter heating at 1200.degree. C. (Japanese publication of examined patent application No. 46995 of 1980; FR No. 2,190,764, U.S. Pat. No. 3,853,567 and D.E. No. 2,218,960).
As an infusibilization of melt-spun green fibers as in the abovementioned two methods, the treatment with air and/or moisture is common, and in this case, it is considered that oxygen performs cross-linking between polymers and infusibilization is carried out. However, since oxygen is remaining in the form of SiO.sub.2 in the ultimate shaped articles of ceramics when an infusibilization is carried out according to such processes, it is known that this gives A bad effect upon the strength of ceramics. (Okamura et al; the 187th Nat. Meeting. Am. Chem. Soc.; April 1984.)